Facsimile recording system



Nov. 25, 1941. w. G. H. FINCH 2,263,986

FAGSiMILE RECORDING SYSTEM Filed May 11, 1940 2 Sheets-Sheet 1 Recorder[imiter BY William G. H. Finch.

ATTORNEY Nov. 25, 1941;

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Valiage w. G. H. FINc 2,263,986

FACS IMILE RECORDING SYSTEM 2 Sheets-Sheet 2 7 Filed May 11, 1940 timetime

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' INVENTOR l ATTORNEY William G.H.'Finch.

Patented Nov. 25, 1941 UNITED STATES PATENT OFFICE mcsmm-z aacoanmesgs'rau wimm o. n. Finch, Newtown, Conn. Application May 11, a, SerialNo. 334,431

6 Claims. (01. 178-83) My invention relates in general to facsimilerecorders and more specifically concerns an improved method for directlyrecording electrical picture signals in apparatus employing thediscoloration of chemically treated dry paper.

Heretofore, direct recording facsimile systems employing papersensitized by chemical means, depended upon the variation of the degreeof discoloration with impressed variations in the received electricalpicture signals. Thus the detected and amplified picture currents wereconnected in circuit with a recording stylus, which was in contact witha sheet of this chemically treated paper and which traveled under theinfluence of a scanning mechanism.

The recording sheet, was mounted on-a conductive platen or drum whichwas also in circuit with the output of the picture amplifier. Therefore,currents would flow through the sensitized paper and the degree ofdiscoloration at any particuiar point in the picture would be dependentupon the current flowing therethrough, which in turn would be dependentupon both the intensity of the picture signal, and the electricalimpedance of the paper at that point.

In order tomanufacture recording paper at a price which would not beprohibitive to owners of home facsimile recorders it is necessary to usean inexpensive stock of paper and to automatically and continuouslyimpregnate it with the required chemicals, such as titanium oxide.

It has been found that with even reasonably strict control of themanufacturing process there still are irregularities in the chemicalcoating which varies the electrical resistance of the coating at variouspoints. Thus, a given current when passing through different points ofthe paper does not produce equivalent discolorations.

'Another factor which considerably reduces the fidelity of reproductionin electrochemical recording is the non-linear relation between theintensity of discoloration and the magnitude of the picture signals.,That is, it has been found that even for a homogeneously impregnatedrecording sheet, a variation in current from zero to a predeterminedmaximum does not-cause a corresponding linear variation in tones fromwhite to black on the recording sheet.

Predistortion of the electrical image currents to compensate for thenon-linear characteristic of the sensitized paper has been found to beIn order to eliminate the drawbacks encountered in the recording on drysensitized paper, I have discovered that I may dispense with the methodfor producing the various tones of a picture by employing a single andreproducible con-' stant current intensity to reproduce the entire tonalrange of a picture, by the expedient of varying the spacing betweenrecorded currents of this single intensity in accordance with thedesired shades or picture intensities. I

My novel means for recording facsimile pictures overcomes the drawbacksencountered in the variations of resistance in the recording paper. Ihave found that I need not replace present day transmitting equipmentand may employ all of the mechanical features embodied in pres--difllcult, as no simple-relation exists between the discoloration andthe magnitude of the picture current.

ent day receivers, as I need only add a single circuit in the output ofthe picture amplifier in order to obtain my novel and improved picturerecording means.

My invention contemplates the usual reception and detection of atransmitted picture signal. This detected signal, which may comprise asusual, a current of varying amplitude varying in accordance with thelights and shades of the image being transmitted is then applied to actas a control over the frequency of an electrical impulse generator whichmay be in the form of the well known relaxation oscillator,multivibrator or the like.

Thus, the output of my pulse generating circuit will be a succession ofdiscrete electrical impulses of approximatelyconstant amplitude butvarying in frequency in accordance with the amplitude of the picturesignal. The frequency pictures.

This series of varying frequency electrical impulses are then applied tothe stylus of the recorder, as in the prior art devices, subsequent toamplification of the pulses to a value sufliciently great to insure thateach pulse will record as a uniform deep black spot on the sensitizedpaper, regardless of the inherent variations in the electricalproperties of the sheet.

Thus, my recording circuit produces varying intensity images by means ofa series of black, irregularly spaced points on the recording paper.Furthermore, I provide means to prevent the amplitude of the individualelectrical impulses which comprise the picture currents from obtaining amagnitude suiiicient to cause deterioration or perhaps burning of therecording paper.

It is therefore an object of my invention to provide a facsimilerecorder which can reproduce the full tonal scale of an image and whichis independent. of the heterogeneous electrical characteristics of therecording paper.

A further object of my invention is to provide for a facsimile image inwhich the various half tones are produced by means of a series ofconstant intensity diversely spaced pots.

A still further object of my invention is to provide means for recordinga facsimile image on chemically treated recording P per by a series ofelectrical impulses amplified sufliciently to insure uniformity of therecorded spots.

Another object of my invention is to provide means for recording afacsimile image with large intensity electrical impulses, while insuringthat the recording paper will not be burned.

These objects and others become evident from the following specificationtaken in connection with the accompanying drawings in which,

Figure 1 is a schematic diagram of the recorder of my invention.

Figure 2 is a graphical illustration of the wave form of the electricalcurrents in the impulse generating circuit, and,

Figure 3 is a graphical illustration of the wave form of the currents inthe circuits of my facsimile recorder.

The embodiment of my invention illustrated in Figure 1 records thereceived picture currents on a sheet of dry chemically treated papermounted uponascanningunitwhichmaybeofthetypes illustrated in my PatentNos. 2,141,975 and 2,036,128. In these units, a stylus continuouslyscans the recording paper under the influence of a mechanical drive,while the picture currents are passed through the paper to a conductiveplaten.

My recorder ll may employ any of the above mentioned devices, or othersknown in the art, to record the picture currents subsequent to theirpassage through my novel controllable impulse circuit which wfllhereinafter be completely de- The picture signals are received over theordinary communication channels such as telephone lines or modulatedradio frequency carrier currents, and when detected in my detector If,as

required by the particular transmission means, comprise currents ofvarying amplitude as indicated in Figure 3a. Accompanying the picturecurrents are corresponding synchronizing signals which occur in theinterval between scanning lines, and which are of the type more fullydescribed in my copending application Serial No. 203,222, filed April21, 1938. The synchronizing signals do notpass through the recordingchannel, but are switched to the synchronizing means by means of a camoperated by the stylus driving mechanism.

The picture signals when detected vary in amplitude in accordance withthe lights .and shades of the image at the transmitter, and in therecording" devices described in the hereinabove mentioned patents, thesecurrents are directly coupled to the recording stylus and platen.

The discoloration produced in the process of aline by line scanning ofthe recording paper will result in a completed picture similar to thatat the transmitter.

In order to overcome the previously described diiiiculties encounteredin the recording process,

' I first amplifymy picture currents in the ampliglow tube such as neontube.

fler I 3, indicated schematically inFigure 1, and then employ thesepicture currents to serve as a control over an impulse generator, theoutput currents of which are impressed upon the recordmember.

The impulse generator is designed to generate currents of relativelylarge magnitude for a short interval of time, which when impressed uponthe recording paper through a scanning stylus will.

record as a series of sharply defined black points. These distinctpoints, as will be pointed out in the following, may be controlled toproduce a line of the transmitted picture by varying the spacing orfrequency thereof. 7

The circuit which I employ for generating these distinct electricalimpulses is illustrated schematically in Figure 1. However I need not belimited to this embodimentbut may employ any of the well known impulsegenerating circuits in my novel combination for producinga facsimileimage. Thus, although I have indicated a glow tube relaxation oscillatorcoupled to a resistance-capacitance circuit for the generation of theseimpulses I may employ thyratron or multi-vibrator circuits. However, Ihave chosen a glow tube circuit for schematically illustrating myinvention, in order to facilitate the following description.

It is well known that electrical impulses may be generated from theoutput of a relaxation oscillator circuit, which comprises aresistance-capacitance circuit controlled by a gas discharge or Thus, ifa constant source of electromotive force is impressed upon a circuitcomprising a resistance and condenser in series, the voltage across thecondenser will "rise exponentially with time, and approach the sourcepotential as a limit. By suitably d signing the constants of thisresistor and condenser and by paralleling the condenser with a glow tubeas for instance, a neon tube or the like, a series of oscillations, morecommonly referred to as "relaxation oscillations, will be produced.

The oscillation of such a circuit may be explained briefly, y notingthat a glow tube will "break-down or offer a very low impedance to anelectrical current when the potential impressed thereupon reaches thatrequired for the ionization of the gas contained between the electrodes,and will thus almost completely discharge the parallel condenser. Thedeionization or extinction voltage of the glow tube is less than thebreak-down voltage thereof, and therefore the condenser wfll dischargerapidly through the tube until this lower value of extinction voltage isattained.

Immediately after deionization the charging to rise from the extinctionto the ignition point,

which in turn isrdependent upon the characteristics of glow tubeemployed and the values of the impressed electromotive force, resistanceand capacitance.

The approximately triangular alternating wave of suitable magnitude.

cal slope 25 of the voltage wave.

. aaoacsc generated by such an oscillator, when appro-- priatelyconnected in circuit with a resistor and condenser in series willgenerate current pulses of reasonably large amplitude and for a shortinterval of time. A

This basic impulse generating circuit has several disadvantages; themost notable being the exponential, non-linear rise in voltage. Theimpulse generating circuit illustrated in Figure 1 is a modification ofthe basic circuit hereinabove described, in that it provides for alinear rise in voltage between the extinction and ignition potentials ofthe glow tube employed.

To obtain this linear rise I charge the condenser which is paralleled bythe glow tube Ii, through a constant current source, which inthisembodiment is a high-vacuum screen grid tube I8.

It is well known in the art that a screen grid tube such as the pentodel8 will supply a constant current for a wide swing in plate potentialwhen the control and screen grids are maintained at constant potentials.

Thus, to fulfill thes requirements I maintain the screen grid at aconstant potential above the cathode equal in magnitude to that of thebattery l1, and the control grid at a suitable negative potential withrespect to the cathode determined by the battery 2|. This potential isapplied to the control grid through the resistor 22 in order tosafeguard against currents of large amplitude in the grid circuit.

The condenser I4 is charged by the plate current derived from the seriesbatteries l1 and 23 As the tube 16 is of the pentode type, thesuppressor grid is maintained at a very low potential which in thisinstanceis the cathode or ground potential.

when this circuit is in operation, the condenser is charged by the platecurrent flowing through the pentode it, which remains at a constantvalue regardless of the diminishing potential between plate and cathodedue to the increasing potential across the condenser Il. The

' variation in plate potential of the pentode I6 is limited so that theplate current remains at a substantially constant value throughout thecharging cycle. The charging cycle of the condenser is indicatedgraphically by the sloping line 24 of Figure 2a.

When the voltage across the condenser ll reaches the ignition or breakdown potential of the glow tube IS, the tube will conduct a current andit is characteristic of these tubes to ofier a very low resistanceduring the conducting cycle. Accordingly, condenser l4 will rapidlydischarge through the glow tube until the deionization potential of thetube is reached. This rapid discharge is indicated in Figure 2a by theverti- Immediately as the condenser potential arrives at thedeionization potential the glow tube again assumes a relatively highresistance and the charging cycle is repeated. Thus, the voltage waveform appearing across the condenser M will assume the saw-tooth"characteristic shown in Figure 2a. The voltage variation of theserelaxation oscillations is the difference between the ignition andextinction potentials of the glow tube l5. 'As this variation inpotential is relatively small, it may benecessary to resort to a gridcontrolled thyratron or gas filled triode for an oscillation generatorof comparatively greater voltage.

' An important advantage of the circuit illusscribed in laterparagraphs. The linearity be- I tween'variations in plate current andfrequency may be proved mathematically as follows:

The electrical charge q on the condenser at any time following theextinction of the tube may be expressed as m I =qo+fi dt (1) where,

as is the charge remaining on the condenser at the instant ofextinction,

i is the charging current, and

dt is a differential increment of time.

If the charging current i remains constant during the cycle; Equation 1reduces to =qo+it (2) This condition is essentially true for theconstant current pentode circuit employed.

The voltage V across the parallel combination of condenser I4 and glowtube l5 at any instant is,

v where,

C is the capacitance of the condenser l4, and

q is the value expressed in Equation 2.

But,

where, Ve is the extinction or deionization potential of the tube, andtherefore,

The frequency of oscillation j is equal to the reciprocal of the periodT, or

Thus the frequency output of the relaxation oscillator is a linearfunction of the plate current of the pentode it which is constant forany particular value of control grid potential. Although the extinctionanddgnition potentials vary slightly with the frequency of theoscillations, the deviations are sufficiently small to be insignificantin this instance.

If the grid voltage-plate current characteristic of the pentode employedis substantially linear,

it is therefore possible to continuously vary the frequency of theoutput of the reiaxationoscillator by means of signals impressed uponthe control grid. The frequency control character- 4 aacaoao A I isticof this circuit will be referred to again in later paragraphs, butfirst, the actual generation of the electrical impulses will beconsidered.-

As indicated in Figure 1 the potential across the condenser ll isimpressed upon a series circuit of condenser 2i and resistance 21.Considering the complete circuit now of the batteries l1 and 23. theparallel combination of condenser l4 and its associated glow tube It.and the circuit comprising condenser 28 and resistor 21, the generationof discrete electrical impulses may be explained briefly as follows:During the charging periodof condenser ll, the glow tube l5 constitutesarelatively high and almost infinite impedance and thus conducts nocurrent. Condenser has a relatively small capacitance and will charge ina relatively small period of time and maintain the current in thecircuit of resistor 21 and condenser 26 at a negligible value.

The potential drop across condenser 26 will according]! be approximatelyequal to the difference between the potential of the series batteriesgreat enough so that a trueblack will be recorded thereby at the pointat which the paper has its maximum 'electricalimpedance, inasmuch asthis will certainly result in a true black at a point of lowerimpedance.

As electro-chemical recording may constitute a me hazard, forlargeamplitude currents, I provide a current limiter 38 connected inseries with the output of the pulse generator, in order to limit theimpulse magnitude to a value below that required to ignite the paper.

This current limiter It may assume theform of one of the devices I-havedescribed in my co- I1 and 23 and the potential drop across thecondenser ll. However, upon the break down of control the magnitude ofthe generated impulses.

Furthermore this battery may be omitted it direct coupling is employed.

The electrical impulses are amplified by the vacuum tube 3| whichutilizes battery 34 as a source of energy and thus the voltage appearingacross the load resistance 33 is of the form indicated in Figure 21).That is, the amplified impulses 35' are of relatively short duration andoccur at approximately the time of the discharge of the glow tube l5,graphically illustrated as the vertical drop 25 in voltage, Figure 2a.

The duration of the individual impulses 35 indicated in Figure 2b iscontrolled by the relative magnitude of the constants of condenser 28,resistance 21 and the potential applied to the circuit.

- rectly in accordance with the amplitude of the In order to generate animpulse 35 of a magnitude sufficient to record on dry recording, paper,it may be necessary to employ several stages of amplification, or it maybe preferable to first amplify the relaxation oscillations indicated inFigure 2a by means of a suitable amplifying device such as aresistance-capacitance coupled or each will record as a distinct spot ofthe maximum possible density obtainable on the various dry recordingpapers known, regardless of the inherent diflerences in the electricalcharacteristics, of the recording'paper. Accordingly, the

magnitude of these electrical impulses must be.

scanning stylus.

are impressed upon the control grid of the'pentode l6, as indicatedinFigure 1, and if the plate current grid voltage characteristic issufiiciently linear, the frequency of the relaxation oscillations andthe corresponding impulse frequencies of substantially constantamplitude will vary dipicture current.

This result is indicated in Figure 3 wherein two cycles of the picturecurrents are illustrated by Figure 3a. The corresponding variationsin-frequency of the relaxation oscillations are indicated in Figure3b-and it may beseen that the increased frequency corresponds toincreasedamplitude of the picture current. The electrical impulsesappearing across the amplifier load resistance II are also illustratedin Figure 3c. These impulses are at a frequency equal to that of therelaxation oscillations illustrated in Figure 3b, and are connected incircuit with the recordingstylus after first passing through the currentlimiter ll.

It is important that the stylus be moved at a constant velocity acrossthe recording sheet in order that the spacingbetween electrical impulsesbe determined solely by the frequency thereof. 7

, The maximum-frequency of these electrical impulses will determine theblack or the densest portion of the recorded image, and this frequencywill be a'function of the speed of the That is, if one facsimile system,such as my high speed printer" and transmitter, operates the stylus at arelatively higher speed than another, it may be necessary to design theimpulse generating circuit to operate at a higher frequency in orderthat the maximum number of black impulses appearing per inch of scanningline on the recording sheet shall be the same. I

In order to secure a clear white or the natural color of the recordingpaper, I mayreduce the frequency of the discrete electrical impulses tozero by negatively biasing the control grid of pentode l6, by means ofbattery 2|, so that the plate current at no signal will be equal tozero.

The maximum frequency which may be employed is determined by thedefinition of the recording sheet. Thus if the stylus is moved relativeto the recording paper at a velocity of ten inches per second, and it isdesired that the densest of the portion of the picture contain fivehundred distinct impulses per linear inch, I adjust the maximumfrequency of my oscillator to five thousand cycles per second.

If as previously mentioned, the pentode l 8 providing the condensercharging current, is biased I to cut off, I will obtain a linearvariation in frequency. from zero to the maximum of the impulsesgenerated, with respect to the amplitude of the incoming signal and willthus secure a picture containing the full tonal scale of the a originalat the transmitter. Although, it is possible to obtain a pure whitebybiasing the pentode It to cut off, it may be desirable to operate with aminimum frequency of perhaps thirty or flfty points per inch, as thiswill produce a picture that simulates a screened half-tone. Also, thevery low frequencies, which may print five or ten points per linearinch, may not-be easily accommodated by the eye because of therelatively large spacing. The minimum frequency desired will bedependent upon the speed of printing, and may be obtained by ad-'lusting the control grid biasing battery 2| of the pentode l6.

It is important to note that I obtain my improved picture withoutresorting to changes in receiver for receiving currents varying inamplitude in accordance with the lights and shades of a picture to berecorded; said receiver including means for translating saidreceived'currents of varying amplitude into signals of correspondingvarying frequency; a recording sheet responsive to electric currentsflowing therethrough for recording images in accordance with saidcurrents; and means for impressing said currents from the output ofsaidreceiver across said recording paper.

2. A facsimile recording system comprising a receiver for receivingcurrents varying in amplitude in accordance with the lights and shadesof a picture to be recorded; said receiver including means fortranslating said received currents of varying amplitude into signals ofcorresponding varying frequency and substantially constant amplitude; arecording sheet responsive to electric currents flowing therethrough forrecording images in accordance with said currents; and

- means for impressing said currents from the outthe transmittingequipment now installed and that all of the mechanical features ofthe'-pres-- ent day facsimile recorders may be employed with most of theassociated electrical circuits. I need only add a final stage which isan impulse put of said receiver across said recording paper. a

3. A facsimile recording system comprising a receiver for receivingcurrents varying in amplitude in accordance with the lights and shadesof a picture to be recorded; said receiver includgenerator of the formindicated in Figure 1. If

the facsimile receiver already contains acurrent limiter I need add noadditional equipment other than the impulse generator and need onlyadjust the current limiter to limit the currents flowing therethrough toa magnitude less than that which may possibly constitute a fire haz-vard when passing through the record sheet.

My improved-recording means allows me to employ a relatively inexpensivegrade of paper for recording purposes. However, with improvements in therecording paper and increased definition thereof, I will be able toemploy still quency above the carrier to correspondingly deviate thefrequency of my impulse generator which is coupled to the recordingstylus.

Although ordinarily in facsimile recorders a positive picture isproduced directly by the discoloration of the recording sheet andincreased amplitude increases the intensity of the discoloration, I mayelectrically invert the image, as is well known in the art, to produce anegative record image, if required.

While I have described one embodiment of a means for faithfullyreproducing facsimile image without dependency upon the characteristicsand quality of the recording sheet, many'other modiflcations of thisbasic idea may be devised by those skilled in the art.

Therefore, although I have illustrated a basic type of impulse generatorand means for controlling the same, I do not wish to be limited to thisparticular modification, but prefer that the scope of my invention beconsidered as that covered by the appended claims.

I claim:

ing means for translating said receiver currents of varying amplitudeinto signals of corresponding varying frequency and substantiallyconstant amplitude; a recording sheet responsive to said currents ofsubstantially constant amplitude and variable frequency for recordingimages in accordance with said frequency variation; means foroperatively effecting said recording sheet by said currents, and meansfor obtaining a uniform intensity of all said recorded currents.

' 4. A facsimile recording system comprising a conductive platen. and asheet of electro-chemically sensitized recording paper in juxtapositionwith said platen, means for scanning said sensi-" tized paper with aconductive stylus, a receiver for detecting and amplifying transmittedsignals varying in amplitude in accordance with the lights and shades ofa picture, and means for translating said variable amplitude picturecurrents into discrete electrical impulses, said impulse currentfrequency being directly proportional to the amplitude of said variableamplitude picture currents, and circuit means for impressing saidimpulse currents between said conductive platen and stylus, toreproducesaid transmitted picture.

5. A facsimile recording system, comprising a conductive platen, and asheet of electro-chemh cally sensitized recording paper in juxtapositionwith said platen, means for scanning said sensitized paper with aconductive stylus, a receiver for detecting and amplifying transmittedsignals varying in amplitude in accordance with the lights and shades ofa picture, means for translating said variable amplitude picturecurrents into discrete electrical impulses, said impulse currentfrequency being directly proportional to the amplitude of said variableamplitude picture currents, and circuit means for impressing saidimpulse currents between said conductive platen and stylus, theamplitude. of said impulse currents being sufficiently great to insureuniformity of all said recorded impulses.

6. A facsimile recording system, comprising a conductive platen, and asheet of electro-chemically sensitized recording paper in juxtapositionwith said platen, means for scanning said sensi- 1. A facsimilerecording system comprising aas tired paper with a condu ve stylus, areceiver impulse currents between said conductive platen and stylus, theamplitude oi. said impulse currents being sufllciently'great to insureuniformity of all said recorded impulses; and means in circuit with saidstylus for precluding the burning of said recording sheet.

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